User equipment (UE), such as mobile phones, must be able to maintain a level of service with a communications network as they move in and around the communications network. In this regard, mobility techniques such as cell reselection (where the UE is either in the so-called radio resource connection (RRC) idle mode or in a “CELL_FACH” state in RRC semi-idle mode) can be used to maintain connectivity with the network.
In general, when the UE is camped on a cell, it regularly searches for a better cell according to cell reselection criteria. The cell reselection process enables the UE to “reselect” a neighbour cell as its new serving cell, where the neighbour cell has been determined to provide a better quality or stronger signal than the current serving cell. More particularly, the UE switches to a neighbour cell if the signal strength of that neighbour cell has been determined to be better than the signal strength of the current serving cell for at least as long as a “time to trigger” threshold time period specified by the network. A timer is therefore used to measure the amount of time that the neighbour cell signal strength remains above that of the serving cell signal strength. This timer is reset to zero when the neighbour cell signal strength drops below that of the serving cell. The timer is started again once the signal strength of the neighbour cell rises above the serving cell signal strength a second time.
In some situations, the signal strength of the current serving cell may drop and there may not be any neighbour cells available for cell reselection (i.e. either because no neighbour cells have been detected that are in proximity to the UE or because the signal strengths of the existing neighbour cells do not satisfy the cell reselection criteria). In this case, if the signal strength of the current serving cell drops below a minimum service level specified by the network then the UE determines that it is “out of service” and thus is unable to maintain communications with the network. The UE can then start a service recovery procedure in order to regain a level of service, for example, by initiating a public land mobile network (PLMN) search.
In such an out of service mechanism, the UE is determined to be out of service if the signal strength of the serving cell falls below the minimum service level for at least as long as an out of service threshold time period. This threshold time period is specified in 3GPP as 12 seconds for idle mode and 4 seconds for FACH mode. A timer is therefore used to trigger the start of the service recovery procedure. The UE must re-select to a suitable cell before the out of service time period expires. A suitable cell is deemed to be a cell which meets the network specified minimum service criteria. The timer is reset to zero when the serving cell signal strength rises above the minimum service level. The timer is started again once the serving cell signal strength falls below the minimum service level a second time. The timer expires if the signal level of the current serving cell stays below the minimum receive level for the out of service threshold time period.
Therefore, only measured signal strengths that are consistently either above or below a reference threshold signal level for a predetermined time period are taken into account in order to perform either a cell reselection or an out of service procedure.
The signals received by the user equipment may be affected by interference between the cells and from other transceiving (i.e. transmitting and/or receiving) sources and also the urban environment in which the UE is operating. This may give rise to spurious signal strength readings (i.e. an unexpected reading that does not follow the trend of the previous readings). For example, in cell reselection, such spurious signal strength readings can cause a timer to reset to zero when in fact the average of the neighbour cell signal strength readings are stronger than that of the service cell, and therefore the timer should not be reset. The resetting of the timer results in an unwanted delay for performing, for example, cell reselection.
A need therefore exists to overcome the aforementioned problems.